Vitamin B-12 deficiency is more common with increasing age and affects 5 to 15 percent of adults. Some of this is early deficiency and not associated with symptoms. Once symptoms develop, they are sometimes overlooked because they develop slowly and are similar to many other problems related to growing older. It pays to be alert to symptoms and consider taking a vitamin B-12 supplement or eating food fortified with vitamin B-12. Vitamin B-12 is one of the few vitamins Mayo Clinic experts routinely recommend for older adults.

Vitamin B-12 plays a role in red blood cell formation, cell metabolism, nerve function and bone health. The primary natural sources of vitamin B-12 are meats, fish, shellfish, eggs and milk; it is not found naturally in plant products. Some foods such as breakfast cereals are fortified with vitamin B-12. It is stored in large quantities in the liver and recirculated in the body, so even if you suddenly stopped getting vitamin B-12, it would likely take years for deficiency to develop.

Deficiency can occasionally occur due to a diet that contains very little vitamin B-12, such as with strict, long-term vegans (vegetarians who consume no animal products at all) and people who are malnourished, such as some who abuse alcohol. However, it most commonly develops because the body isn’t properly digesting and absorbing vitamin B-12 due to causes that include:

Aging — With age, the ability to absorb vitamin B-12 often diminishes. Inadequate nutrition and higher risk of digestive diseases also may be an issue.

Diseases that affect digestion — The most common of these is pernicious anemia, in which your own immune system destroys a protein that enables you to absorb B-12. Celiac disease and Crohn’s disease also can interfere with proper vitamin B-12 absorption, as can gastric bypass surgery.

Prolonged use of certain medications — Proton pump inhibitors, such as omeprazole and H-2 blockers, which reduce stomach acid to treat gastroesophageal reflux disease (GERD) and stomach problems, can interfere with vitamin B-12 absorption. The diabetes drug metformin also is associated with lower vitamin B-12 levels.

Symptoms of B-12 deficiency can be subtle at first. Early symptoms may include a persistent tingling or prickly feeling in your feet or hands, weakness, numbness, imbalance, and mental problems such as confusion, depression, irritability and forgetfulness. Rarely, vitamin B-12 deficiency is a cause of dementia.

As vitamin B-12 deficiency becomes more advanced, a shortage of red blood cells (anemia) or even white blood cells and platelets may develop. Deficiency often is caught before it reaches this level.

Diagnosis of vitamin B-12 deficiency typically involves blood tests. Testing for anemia may be one of them, but just because you’re not anemic doesn’t mean you’re not vitamin B-12 deficient. If vitamin B-12 deficiency is diagnosed, additional testing is sometimes performed to look for an underlying cause.

For those who are deficient, raising vitamin B-12 to adequate levels may first involve correcting or treating an underlying problem, if possible. People with severe deficiency or who have an underlying problem in which the body poorly absorbs vitamin B-12 may need injections of high-dose vitamin B-12. These injections may be given several times a week at first, reduced to weekly injections, and then monthly injections if the underlying cause can’t be corrected.

Taking high-dose oral vitamin B-12 may sometimes be an alternative to injections; however, it may be better used as long-term maintenance therapy after vitamin B-12 levels have been stabilized.

The body responds rapidly to high-dose vitamin B-12, with nerve symptoms subsiding over weeks to months. However, the degree of improvement of nerve problems is typically influenced by how severe and long lasting the nerve problems were before treatment. In advanced cases, nerve problems can be irreversible.

If you eat meat or eat breakfast cereal fortified with vitamin B-12, you’re likely getting the recommended daily intake of 2.4 micrograms (mcg). However, older adults may not be absorbing this B-12 efficiently and may want to take a daily supplement to prevent deficiency. You can’t overdose on vitamin B-12, as it’s not toxic. If you have a disease or take a drug that increases risk of vitamin B-12 deficiency or eat a strict vegan diet, talk to your doctor about appropriate supplementation levels or screening for deficiency.